Balance weight



Patented `Iune 24, 1930 PATENT orFce WALTER. 0. SNELLING, OF ALLENTOW'N,PENNSYLVANIA IBALANCE WEIG-HT Application filecl December 8, 1928.Serial No. 324.686.

My invention relates to improvements in standards of mass such as areemployed in wveighing operations, and inore particularly my inventionrelates to improved balance vweightsv of the type commonly used bychemists and physicists in weighing operations in which a high degree ofaccuracy or precision is required. My invention also relates to a newcomposition of matter from which weights of. novel and advantageous typemay be made. ventionis to provide ineans by which balance weights ofpermanent nature and of an'exceptionallyhigh degree of accuracy may bemanufactured at a relatively low cost as compared with manufacture bymethods at present known.

' In order to satisfactorily fulfill the normal functions of a standardof mass or weight, a balance weight must be capable of being calibratedto a high degree of equivalency with an ultimate standard of mass orweight, and after being so calibrated must remain constant, withouteither' further loss or gain Aof weight within the limits' of accuracythat are required.

Although it is possible by methods now known to prepare weights for thecoarser loperations of weighing` which are entirely satisfactory forpractical purposes, it has been found very difficult to prepare weightsthat are entirely satisfactory for the more refined operations ofprecision weighing. Of all lof the metals at present available for themanufacture of weights, platinum has been found to possess the mostsatisfactory resistance to oxidation, and has accordingly been employedinv the preparation of balance weights where the highest degree ofaccuracy is required, but as the cost of platinum has been for manyyears greater than the costhof an equal Weight of pure gold, the use ofplatinum for the heavier weights of commercial sets has beenimpracticable for economic reasons.

As a result of the high cost of platinum, making its use impractical inthe construction of .the sets of balance weights ordinarily used bychemists and physicists in their One object of my in-v every-day work,platinum has been used as the material of construction for ultimate ornational standard of weight or mass, while brass weights have beenemployed and are generally used in most ordinary work.

This almost universal use of brass as the material of construction ofcommercial units of weight or mass has led to an anomalous situationwith respect to correction for buoyancy. Although a brass kilogramweight will exactly balance a platinum kilogram weight when the twoweights are compared in an absolute vacuum, the two weights will notexactly balance each other when weighed in air. Since every substancepresent within a fiuid is buoyed up by a weight corresponding to theweight of a volume of the fluid which it displaces, it will be evidentthat a brass weight of any given volume when weighed in air will bebuoyed up to the extent of the weight of the volume of air which theweight displaces, and similarly a platinum weight when weighed in airwill be buoved up to the extent of the weight of the volume of air whichit displaces. The volume of a kilogram of platinum is `less than halfthe volume of a kilogram of brass, and accord-v ingly two kilogramweights, one made of brass and one lmade of platinum, will have I quitedifferent Volumes, and although each of the two weights will possessexactly the same number of units of mass, and will accordingly balanceone another when weighed in a vacuum, their apparent weight, whenweighed in air, will be quite different. Because of the adoption ofplatinum as the material of construction of the ultimate standards ofweight and because of the unfortunate difference between the specificgravity of platinum and the specific gravity of brass, which lattermaterial vhas bv common usage become the material of coifstruction ofprecision units vof mass for commercial purposes, it is tcustomary toapply a buoyancy or vacuum correction to all weights that are used inprecision weio'hino's as referred to the platinum ultimate c' stanllard,and a corresponding correction would have to be made if weights of anmaterial having a lower or a higher speci c gravity than that of brasswere to, be used in precision weighings that would be com ared withweighings made with brass weig ts.

Although brass tarnishes relatively slowly, when exposed to atmospheric.conditions, it is well recognized to slowly oxidiae, with very apparentdarkening of the surface, and wit an increase in weight which isentirely too high to be disregarded in Vprecision weighings. Recent work(Proc. Ph s. Soc.,

London, Apr. 15, 1928 Ab. Jour. ranklin Inst., voi. 206, No. 4, ct.1928, .564) has alforded quantative figures for t e corrodibility ofbrass when exposed to atmospheric agencies for even short periods oftime, but it has long been recognized that brass weights slowly undergosuperficial changes with alteration of weight of suflicient importanceto decidedly influence weighings of precise character. Accordingly,brass weights for precision weighing are usually covered with asuperficial coating of lacquer or a plating of gold or platinum for thepurpose of reducing atmospheric deterioration, and wei hts of thisgeneral character have now, by ong usage, been accepted as a secondarystandard in all precision v weighing operations. Correction for buoyancyis only made in work where the very highest accuracy is required, thegeneral assumption being that even in precision operations the weight asstated is the apparent wei ht in air of the object weighed as balanceagainst balance weights having an average specific gravity of 8.4, whichis lthe average specific gravity of brass.

I have discovered that by incorporating in suitable amounts anagglutinant or cement such as the well known thermo astic condensationproducts ofphenol an formaldehyde with a linely divided solid of highdensit such as tungsten powder obtaine by the re uction of the oxide, Ican obtain'a product having substantially the specific gravity of brass,and efl'ering many advantages over brass as a material for theconstruction of balance weights. p I

As an exam le of my present invention I willdescribe-t e'method which Iprefer to employ in making a gram balance weight from an admixture of acondensation duct of phenol and formaldehyde of the akelite type andpulverulent metallic tungsten such as may be obtained by the reductionof p ltungstic oxide.

As the density of the kind of brass geni erally used in the manufactureof weights is 8.4, and as this figure has been accepted by theBureau ofStandards oftheDepartment of Commerce of the U. S. Government as thecommercial standard for the density of standard weights of recision, itwill be evident that the over-a volume of a composition weight havingthe same density as a 100 gram brass weight will be obtained by dividing100 by 8.4, and by making this calculation it will be found that therequired volume is 11.905 c. c. In my composition weight it will ofcourse be evident that the combined wei htof the `metallic t sten filerand oft bakelite. mtl'iltlwillbb 100 grams, while the respectiveproportions of akelite and tungsten powder must be such that the densityof theresulting product will be 8.4. If a: be taken as the volume incubic centimeters of the bakelite to be used in making the 100 grainweight, and if be taken as the volume of thev tungsten present in finelydivided condition as tungsten powder, then it will be evident that o andusing bakelite that has a specific gravity of 1.25 and tungsten having a.specific gravity of 18.77, it will also beevident that,

1.25z 18.77y--f100.6QQ, since the volume of tha bakelite when multipliedby its specific gravity when added to the volume of the tungsten when.multiplied by its specfic gravity, must represent all of the mass thatis present in the weight, and.

accordingly must total 100 grains.

Solving the equations it willbe found that in the present example a:equals 7.046 c. Vc.

and that y-equals 4.859 c. c. As partshy`` weight are usually desired inthe makin up .:f

of the thermoplastic composition, the a ve volumes are next changed to wi hts, by multiplying each by the'corres ading s cific gravity. Byperforming ese calcu ations it will be found that 8.807 parts b weightof bakelite, or 59.2% by volume, wit

91.203 parts by weight of tungsten powder,`

or 40.8% by volume will 've a resulting compound that will have t e samespecific gravity as, bras, and that ma accordingly be used to formweights whic may be em-` ployed in weighing `Leperations with brassweights without requiring volume or buoy-I incomplete reduction oftheoxide." Bakelite i also varies somewhat in density, and accordingly,'in4the ,preparation of' weights in ac'-4 cordance with my presentinvention, Ilfiiid it desirable to use a mixture'of tungsten powder andVbakelite containing slightly more than' 40.8%" of tun ten by volume andslightly less: than 59. of bakelite by volume, so as to obtain a basecomposition having a specfic gravity slightly' higher than `fication,Figure l is a representation of a vertical cross section through aweight made in accordance lwitli iny present invention when using acomposition adjusted to have a density of 8.4. Figure 2 is acorresponding cross section through a weight con- 'structed of acomposition having a true density slightly in excess of 8.4. In Figure2, l is the body of the weight, which is provided with a conical orfrustro-conical opening in its bottom. 2 is a frustro-conical plug,preferablyv made of the same composition as 1, and capable of engagingwiththe walls of the cavity of l as shown. is a cavity or chamber, byvarying the size of which the apparent density of the composite weightmay be controlled. The plug 2 may be adhesively secured in place in thebody l, or may be elasticallyheld by compression by being forciblypressed into position. An air-tight or hermetic seal is desirable..

Although I have referred to bal-:elite as a preferred phenoliccondensation product, it will be evident that many equivalents inay beused instead of this particular plastic. The basis of iny invention isthe production vof'a chemically inactive' coinposition from a matrixmaterial and a filler inaterial in the form of a finely divided powderof high density, to form a solid having substantially the specificgravity of brass, and the turning, moulding or forming in any othersuitable way of balance weights vfrom this composition. As a matrixmaterial I may einploy any suitable synres, which term will be used asan embracive designation of the large group of thermoplasticcondensation products of phenol, urea, phthalic acid, etc., withaldehydes, alcohols, Sulfur chloricle, etc., or instead of atherinoplastic condensation product as my matrix may employ any othersuitable agglutinating or cementing agent. offering suitable resistanceto atinospheric deterioration. -The requirements of a suit-able matrixmaterial are hardness and toughness in the final polymerized,

set, hardened or reacted condition, a high cementing Value forpulverulent materials and a liigh degree of resistance to atmos- .phericagencies such as 4oxygen and moisture, and in the moulding art a verylarge number of materials are known which may be employed withpulverulent fillers to form, after suitable hardening treatment, hard,tough solids of chemically inactivel nature suitable for the presentuse. As my filler material I inay employ any powdered metal having aspecific gravity of 9 or more, but I recognize as equivalents ofpowdered metals aiiy me- 'tallic compounds that possess the desired highspecific gravity. Tungsten carbide is an example of a nonmetallicsubstaiice capable of use in pulverulent condition as a filler for usewith a suitable matrix material, and other metallic carbides, oxides,borides, silicides, etc., inay be used.

Tlie material which I prefer to employ in the practice of my presentinvention is a coinposition made by admixing approximately equal partsby volume of iinely divided metallic tungsten and pure uncured orintermediate moulding bakelite free from Wood pulp or other of thecustoinary fillers used in the moulding art. The resulting compositionmay be directly pressed and moulded into cones and cases as shown inFigure 2 of the drawing and possesses a true density sufficiently above8.4 to permit of satisfactory adjustment of both density and weight to'be made of the finished weight by the removal by attrition or by anyother suitable means of any required quantity of thefrustro-conical.plug necessary to accurately calibrate the finishedweight both with respect to true weight and apparent density. Materialmay be removed from either the upper or the lower flat surface of thefrustro-conical plug, the removal of material from the upper fiatsurface decreasing the weight of the finished weight without changingits over-all volume, while the removal of material from the lower flatsurface decreases both the weight of the finished weight and i tsover-all volume. By always making the combined weight of the body andthe plug slightly in excess of the desired finished weight, andcompleting the calibraltion from either the upper or lower surface ofthe frustro-conical plug as may be necessary to give the desiredfinished density, it is a simple matter to calibrate weights made inaccordance with the present invention both as to true weight and truevolume on the basis of comparison with customary brass weights.

Although it is desii'able that the apparent density of my finishedbalance weight should be 8.4, in order that no buoyancy or volume.correction may be necessary when weighings are compared with weighingsmade with brass weights, I find it desirable to einplov a mixture'ofmatrix material and filler material wliich has a density of from 8.5 to10.0, in order to provideisuitable excess density to allow for thepresence of a cavity within the weight for Calibration purposes.Altliough'the percentage of matrix material and filler material which Iemploy will of course vary with the nature and density of the particularmaterials used, in general I employ from 5% to 20% by weight of acementitious matrix-material having a density of 5 or less, and froni80% to 95% by weight of a pulverulent filler material having a densityof 9 or more.

,usage and exposure to unfavorble atmos-. pheric or chemical agenciesmuch better than invention.

Weights made in accordance with my prescnt invention possess markedadvantages over' brass. weights in resistance to atmospheric agencies,and accordingly in accuracy.l after longv riods of use. Weights ..made'by a moul 'ng operation as herein described are initially cheaper topproduce than brass weights-and may be used with brass weights withoutinvolving any buoyi anoy or volume corrections, and will maintain theiraccuracy under conditions of hardl the best brass weights that can bemade by methods at resent known. The ability to withstand la oratoryfumes without deterioration isa most vimportant advantage ofweightsinade in accordance with my present Instead of usingpowderedmetallic tungsten as my high density filler material I may employ anyother suitable owder havinga density in excess of 9. pongy lead or.powdered lead may be used to advantage in the preparation of weights inaccordance with my present invention, but is less desirable thantimgsten powder because of cific gravity of brass composing a homothetendency of lead to slowly oxidize upon exposure to air and moisture. Inone form of- .my present invention I prepare a thermoplastic compositionfrom powdered lead Vand bakelite, form this composition into weightbodies slightly heavier than the desired jfiial wei These weight bodiesare next trea which dissolves the minute amount of lead which is presentat the exposed surface of the weight.l After careful washingto removeadsorbed nitric acid-and lead nitrate,

'the. weight body is calibrated to bringvit to the correct fin'alweight. Although I fer i with a solvent for 'lead such as dilute nitricacid, for example, n ,n

maam;

present at the exposed surface of the weight by a solvent as described``Asequivalent to the described process I may coat or cover mycomposition weights with a thin film of any suitable protecting agent ofwhich I find phenolic condeasation product laequers form the mostsuitable agents.

I am aware that moulding compositions have long been made by theadmixture of bakelite and other synres compositions with a wide varietyof filler. materials such as wood pulp, preeipitated chalk and likematerials, but no one has earlier attempted to. make, or has seen anyadvantage in making, thermoplastic mouldable compositions. from a matrixmaterial suchas bakelite and a filling material having a specificgravityin excess of 9, in such proportions as will make a composition havi aspecific gravity such that the compositon may be used instead of brassin the construction of balance weights and for other like purposes.

It will be evident that many modifications may be made without essentialnciples of disclosure as herein and accordingly no limitations should beplaced upon my invention except aslinflicated in the appended claims.

c aim:

'Ill

1. A weight having substantially the speg;

geneois admixture of a synres and afinely comminuted metal having aspecific gravity of 9 or more, n

In testimony whereof,l1 have ,heremto subscribed myV name this 5th dayof December 1928.

WALTER 0. SNELLING.

.to-,employ as my filer materiall a pu veru- I lent metal or metallie'coin undhaving a specific gravity inexcess o 9 and which is wholly inertto at-mo ric agencies, it will be evident from theabove that I maylsuccessfully employ a filler materials which is not wholly inei't toatmospheric agencies, and that I may dissolve orremove the minuteamountof the filler material which is Ill III

